linux-kernel - Re: Fast memcpy patch

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Date:	Wed, 23 Nov 2011 13:45:29 +0200
From:	Mihai Donțu <mihai.dontu@...il.com>
To:	"N. Coesel" <nico@...dev.nl>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Fast memcpy patch

On Wed, 23 Nov 2011 12:25:46 +0100 N. Coesel wrote:
> Dear readers,
> I noticed the Linux kernel still uses a byte-by-byte copy method for 
> memcpy. Since most memory allocations are aligned to the integer size 
> of a cpu it is often faster to copy by using the CPU's native word 
> size. The patch below does that. The code is already at work in many 
> 16 and 32 bit embedded products. It should also work for 64 bit 
> platforms. So far I only tested 16 and 32 bit platforms.
> 

Could you run checkpatch.pl on this an fix all the warnings? Or, if you
wish, I could do it for you.

> --- lib/string.c.orig   2010-08-20 20:55:55.000000000 +0200
> +++ lib/string.c        2011-11-23 12:29:02.000000000 +0100
> @@ -565,14 +565,47 @@ EXPORT_SYMBOL(memset);
>    * You should not use this function to access IO space, use
> memcpy_toio()
>    * or memcpy_fromio() instead.
>    */
> -void *memcpy(void *dest, const void *src, size_t count)
> +
> +void *memcpy(void *dst, const void *src, size_t length)
>   {
> -       char *tmp = dest;
> -       const char *s = src;
> +       void *p=dst;
> 
> -       while (count--)
> -               *tmp++ = *s++;
> -       return dest;
> +       //check alignment
> +       if (( (int) dst & (sizeof(int) -1)) != ( (int) src & 
> (sizeof(int) -1) ))
> +               {
> +               //unaligned. This will never align so copy
> byte-by-byte
> +               goto copyrest;
> +               }
> +
> +       //seek aligment (lower bits should become 0). Because
> +       //we already tested the lower bits are equal, we only need
> +       //to test source or destination for matching alignment.
> +       while ( (length !=0) && (((int) src & (sizeof(int)-1 ))!=0) )
> +               {
> +
> +                *((char*) dst++)=*((char*)src++);
> +               length--;
> +               }
> +
> +       //copy words
> +       while(length> (sizeof(int)-1) )
> +               {
> +               *((int*) dst)=*((int*)src);
> +               dst+=sizeof(int);
> +               src+=sizeof(int);
> +               length-=sizeof(int);
> +               }
> +
> +copyrest:
> +
> +       //now copy the rest byte-by-byte
> +       while(length !=0)
> +               {
> +               *((char*) dst++)=*((char*) src++);
> +               length--;
> +               }
> +
> +       return p;
>   }
>   EXPORT_SYMBOL(memcpy);
>   #endif
> 

-- 
Mihai Donțu
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